[BeDLiNt]

<mx:Fade id="showEff" alphaFrom="0.0" alphaTo="1.0"/>
<mx:Fade id="hideEff" alphaFrom="1.0" alphaTo="0.0"/>
...
<mx:TitleWindow id="ttlToolWindow" ... showEffect="showEff" hideEffect="hideEff"... >

Update:
Не дочитал..
Вот примерно так:

			protected function ttlToolWindow_mouseOverHandler(event:MouseEvent):void
			{
				var overEffect:Fade = new Fade();
				overEffect.alphaFrom = 1.0;
				overEffect.alphaTo = 0.5;
				overEffect.target = event.target;
				overEffect.play();
			}
 
			protected function ttlToolWindow_mouseOutHandler(event:MouseEvent):void
			{
				var outEffect:Fade = new Fade();
				outEffect.alphaFrom = 0.5;
				outEffect.alphaTo = 1.0;
				outEffect.target = event.target;
				overEffect.play();
			}
...
<mx:TitleWindow id="ttlToolWindow" mouseOver="ttlToolWindow_mouseOverHandler(event)" 
		 mouseOut="ttlToolWindow_mouseOutHandler(event)"/>